Presently, as a next-generation communication standard of LTE (Long Term Evolution) systems, enhancement of LTE-Advanced is promoted. In LTE-Advanced systems, a carrier aggregation (CA) technique is introduced to achieve a higher throughput than that of LTE systems while ensuring backward compatibility with the LTE systems. In the carrier aggregation, a component carrier (CC) having the maximum bandwidth of 20 MHz supported by the LTE systems is used as a basic component, and it is designed to achieve communication in a broader band by using these multiple component carriers simultaneously.
In the carrier aggregation, user equipment (UE) can use multiple component carriers simultaneously to communicate with a base station (evolved NodeB: eNB). In the carrier aggregation, a highly reliable primary cell (PCell) to ensure connectivity to the user equipment and a secondary cell (SCell) or a secondary cell group (SCG) additionally configured for the user equipment in connection with the primary cell are configured.
The primary cell is a cell similar to a serving cell in the LTE systems and serves as a cell to ensure connectivity between the user equipment and a network. On the other hand, the secondary cell or the secondary cell group is a cell or a cell group configured for the user equipment in addition to the primary cell.
In the carrier aggregation up to LTE Release 10 (Rel-10), as illustrated in the left side in FIG. 1, it is defined that user equipment uses multiple component carriers served from a single base station to conduct simultaneous communication. Meanwhile, in Rel-12, the carrier aggregation in Rel-10 is further enhanced, and as illustrated in the right side in FIG. 1, dual connectivity (DC) where the user equipment uses multiple component carriers served from multiple base stations to conduct the simultaneous communication is discussed. For example, if all component carriers cannot be accommodated in a single base station, it is considered that the dual connectivity can be effectively utilized to achieve a throughput nearly equal to that in Rel-10.
In the dual connectivity, a split bearer is configured. In the case where a master base station or a macro base station (MeNB) is used as an anchor node for distributing the bearer, as illustrated in FIG. 2, the master base station distributes downlink packets received from a S-GW (Serving Gateway) into packets for transmission to the user equipment via its own cell and packets for transmission to the user equipment via a secondary base station (SeNB). If the split bearer using the master base station as the anchor node is configured, as illustrated in FIG. 3, the user equipment has a physical layer (PHY), a MAC (Medium Access Control) layer (m-MAC) and an RLC (Radio Link Control) layer (m-RLC) for the master base station; a PHY layer, a s-MAC layer and a s-RLC layer for the secondary base station; and a PDCP layer coupled to the m-RLC layer and the s-RLC layer. Packets received from the master base station and packets received from the secondary base station are reordered at the PDCP layer, which are then delivered to an upper layer.
Also, according to LTE standard, re-establishment is performed on the PLC layer and the PDCP layer in handover and reconnection. In the re-establishment of the RLC layer, the transmitting side in the RLC layer discards all of to-be-transmitted RLC PDUs (Packet Data Units) while the receiving side in the RLC layer re-assembles as many RLC SDUs (Service Data Units) as possible from the received RLC PDUs and delivers the re-assembled RLC SDUs to the PDCP layer. Also, various timers for use in the RLC layer are stopped and reset, and all various variables are initialized.
On the other hand, in the re-establishment of the PDCP layer, the transmitting side in the PDCP layer retransmits PDCP PDUs, whose acknowledgement (ACK) has not been received at the RLC layer, while the receiving side in the PDCP layer reorders RIC SDUs received with the re-establishment of the RLC layer and RLC SDUs newly transmitted after execution of the re-establishment. Here, the PDCP layer performs different control operations with a reception window for reordering on the RLC SDUs received with the re-establishment of the RLC layer and the newly transmitted RLC SDUs. Specifically, the PDCP layer does not update the reception window for the RLC SDUs received with the re-establishment in the RLC layer if PDCP sequence numbers of the received packets are out of sequence, but the PDCP layer updates the reception window for the newly transmitted RLC SDUs even if the PDCP sequence numbers of the received packets are out of sequence.
See 3GPP R2-131782 in details, for example.